Development of a high-output laser system intended for machining application is remarkable and practical use of the high-output laser system moves forward in various industrial fields from cutting and welding in manufacturing of steel and automobiles to fine drilling of electronic parts, annealing of liquid crystal and semiconductor devices, etc. This is largely owing to higher output of a laser beam and dramatic improvement of performance, quality, and stability.
As the cross-sectional intensity distribution of a laser beam, a Gaussian distribution (single mode) is mainstream. This has a feature that light can be converged up to the theoretical limit (diffraction limit) through a lens. The needs for a uniform intensity distribution and any desired intensity distribution responsive to a purpose rather than the non-uniform Gaussian intensity distribution also increase with the diversification of laser machining applications.
A superposition system of cutting the section of a laser beam into a large number of pieces lengthwise and crosswise and superposing the cut beams at predetermined positions by an optical system for averaging is available as means for providing a uniform cross-sectional intensity distribution; for example, a kaleidoscope or an integrator corresponds to it. Although the integrator can uniform the intensity to some extent by dividing and superposing the section of a laser beam by the structure of a polyhedron, it is known that the intensity falls into degradation like a spike if a laser light source excellent in coherence is used.
On the other hand, an aspherical beam homogenizer and a diffractive beam homogenizer are available as an intensity uniforming system without dividing and superposing a laser beam. In the former aspherical beam homogenizer, the refraction angle of each light beam is controlled by an aspheric surface so as convert a Gaussian light beam density distribution into a uniform distribution. In this case, the aspherical beam homogenizer has a feature that interference does not occur and high uniformity can be obtained because the refraction angle is controlled so that the light beams do not cross; on the other hand, the aspherical beam homogenizer has the disadvantage in that only a uniform beam circular in cross section can be provided because the aspheric surface is rotationally symmetric with the optical axis as the center rotation axis.
In contrast, the latter diffractive beam homogenizer is provided by applying a diffractive optical element (DOE) as a homogenizer. The DOE is an optical element using a diffraction phenomenon of light by forming the optical element on a surface with a relief microstructure pixelated crosswise in micron units rather than using geometrical optics of refraction, etc. It is applied not only to laser machining, but also to optical communications, etc., because of directly controlling the phase of light; applications of beam splitting, beam shaping, and beam homogenizing are possible in laser machining.